Increased consumption and transportation of oil and its products exacerbate the problem of pollution in lakes and oceans. Even with strict regulation of oil transportation, accidents leading to oil spills still occur frequently. Indeed, numerous accidents occur annually, with thousands of tons of oil being spilled into seas, contaminating marine environments and endangering marine ecology. Of the oil that is spilled, a significant fraction of the residual oil is heavier than water and sinks to the bottom; this oil fraction is frequently termed Group V oils, which have a specific gravity exceeding 1.0. These oil residues are typically rich in polynuclear aromatic hydrocarbons (PAHs), which are toxic to aquatic life. Oil spills in coastal waters, harbors, and oil terminals are especially dangerous and necessitate a rapid response when they occur in order to prevent contamination of marine habitats. Therefore, reliable oil-spill detection systems which can locate and map subsurface spillage are needed.
A number of known methods for detecting oil spills are mostly limited to surface oil. It is well known that oils contain fluorophores, i.e., molecules and functional groups that exhibit fluorescence when excited by light of a wavelength absorbed by such moieties. For example, remote laser-induced fluorescence has been developed for surface oil detection. Airborne fluorescence LIDAR (laser-induced detection and ranging), which combines fluorescence detection with ranging techniques, has been developed as well. However, airborne fluorescence methods are generally unable to detect deep underwater oil contaminants. Furthermore, fluorescence methods are susceptible to interference from other fluorophores in water, such as humic compounds, chlorophyll and other plant pigments.
Methods for subsurface detection and mapping of heavy oils include visual observation, cameras and acoustic/sonar sounding methods. Because of the similarity in oil and other natural sediments, these methods, while providing a rapid survey, generally require confirmation by further sampling and analysis, and therefore do not offer real-time results. Sonar frequently does not provide sufficient contrast with the sedimentary sea floor.
Therefore, there is a need for economical methods and instruments to can detect and image heavy-oil residues in real-time with minimal interferences.